Bulk capacitor and method

ABSTRACT

A bulk capacitor includes a first electrode formed of a metal foil and a semi-conductive porous ceramic body formed on the metal foil. A dielectric layer is formed on the porous ceramic body for example by oxidation. A conductive medium is deposited on the porous ceramic body filling the pores of the porous ceramic body and forming a second electrode. The capacitor can then be encapsulated with various layers and can include conventional electrical terminations. A method of manufacturing a bulk capacitor includes forming a conductive porous ceramic body on a first electrode formed of a metal foil, oxidizing to form a dielectric layer and filling the porous body with a conductive medium to form a second electrode. A thin semi-conductive ceramic layer can also be disposed between the metal foil and the porous ceramic body.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/553,508, filed Sep. 3, 2009, issuing as U.S. Pat. No. 8,238,076 onAug. 7, 2012, which claims the benefit of U.S. Provisional PatentApplication No. 61/094,317, filed Sep. 4, 2008, which are incorporatedby reference as if fully set forth herein.

FIELD OF INVENTION

The present invention relates to capacitors. More particularly, thepresent invention relates to a capacitor which is capable of achieving ahigh capacitance density.

BACKGROUND

Technologies for producing capacitors (including electrolytic capacitorsand ceramic capacitors) are being pushed to their practical physicallimit. Traditionally, high capacitance density is achieved either byhigh surface area such as in tantalum electrolytic capacitors or carbondouble layer capacitors. Alternatively, high capacitance density may beachieved by thin high dielectric constant (K) dielectric materials suchas used in multi-layer ceramic capacitors. Despite these advances,problems remain. In particular, the demand for high capacitance densityis ever increasing beyond limits associated with such methodologies.High capacitance density is a highly desirable feature for makingsmaller electronic devices.

What is needed is a capacitor which allows a high capacitance density tobe achieved. It is therefore an object, feature, or advantage of theembodiments disclosed herein to provide a capacitor and a method ofmanufacturing a capacitor which allows a high capacitance density to beachieved.

One or more of these aspects will become apparent from the specificationand claims that follow.

SUMMARY

According to one aspect of the embodiments disclosed herein, a bulkcapacitor includes a metal foil, a semi-conductive porous ceramic bodyon the metal foil, a dielectric layer on the porous ceramic body (e.g.,formed by oxidation), a conductive medium filling the porous body, and aconductive metal layer encapsulating the porous body. According toanother aspect of the present invention, a method of manufacturing abulk capacitor includes forming a semi-conductive porous ceramic body ona metal foil, oxidizing the semi-conductive porous ceramic body to forma dielectric layer, filling the porous body with a conductive medium,and encapsulating the porous body with a conductive metal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a capacitor in accordance with thepresent invention;

FIG. 2 is sectional view showing a portion of a first electrode, a firstceramic layer and a porous ceramic body according to one aspect of thepresent invention;

FIG. 3 illustrates another embodiment of a capacitor in accordance withthe present invention;

FIG. 4 illustrates a mounting substrate in accordance with the presentinvention;

FIG. 5 illustrates another embodiment of a capacitor in accordance withthe present invention; and

FIG. 6 illustrates a methodology according to one aspect of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a capacitor with a high capacitancedensity. The present invention achieves a high capacitance densitythrough an innovative combination of high dielectric constant ofceramics and the high surface area of a porous matrix. In particular, acapacitor may utilize the high dielectric constant of ceramic materialsin combination with a porous ceramic body having a high surface area.

A porous ceramic body is formed on a metal foil. In general, the metalfoil serves as the first electrode. An intermediate ceramic layer mayalso be disposed between the porous ceramic body and the medal foil. Theporous ceramic body is subsequently oxidized. The oxidation can beformed by various techniques such as thermal or electrochemicaloxidation. The oxidation forms a thin high K dielectric layer on thefree surface of the porous ceramic body. The porous ceramic body is thenfilled with a conductive medium such as, for example, a conductivepolymer. In general, the conductive medium forms a second electrode.Subsequent structure can then be added to encapsulate the capacitorstructure and form suitable mounting structures and/or electricalterminations. For example, after the porous ceramic body is filled withthe conductive medium, it can be encapsulated with a conductive metallayer such as, for example, silver.

The resulting combination of high surface area as provided by the porousstructure and the high K dielectric thin layer provided by thedielectric allows for the making of small-size, high capacitance densitycapacitors.

FIG. 1 illustrates a simplified embodiment of a capacitor 10. Thecapacitor 10 includes a first electrode 12 which may be formed of avariety of materials including a metal foil. A variety of metal foilscan be used. The metal foil may be taken from the group of elementsincluding: Ta, Ni, W, Nb, V, Mo, Fe or their alloys. Kovar and Invaralloys are also preferred since their coefficient of thermal expansion(CTE) matches the CTE of the dielectric layer and/or ceramic layer(s).Suitable foils are available from a variety of sources including, butnot limited to: Carpenter Technology Corporation of Wyomissing, Pa.,Cabot Supermetals Inc. of Boyertown, Pa., and HC Starck Inc. of Newton,Mass. As noted above, the first electrode 12 has a CTE selected to matchthe dielectric and/or ceramic element and thereby minimize thermalstress. In a typical embodiment, a CTE of 3-15 ppm is utilized. Avariety of foil thicknesses can be used for example the 20-250 micronrange. A more typical range may be in the 100-150 micron range.

A semi-conductive porous ceramic body 18 is formed on the firstelectrode 12. It is understood that a variety of geometric profiles(primarily dependent on the shape of first electrode 12) can be utilizedwithout departing from the scope of the invention. It is also understoodthat the porous ceramic body may be formed and/or deposited using avariety of processes with a variety of ceramic layers and/orcompositions. In one embodiment, a thin conductive semi-conductiveceramic layer can be deposited on the first electrode 12 prior todepositing the conductive porous ceramic body 18. A variety ofsemi-conductive ceramic materials can be used including barium,strontium titanate (BST). Other suitable chemical compositions includingelements like Nb₂O₅, TiO2, BaCO₃ and SrTiO3 can also be used. Stillother suitable materials can include lead magnesium nionbate (i.e.,Pb₃MgNb₂O₉), lead titante, and perovskite oxides.

FIG. 2 shows a simplified sectional view of an embodiment having a firstelectrode 12, a semi-conductive first ceramic layer 17 and asemi-conductive porous ceramic body 18. It is understood that thethicknesses are not shown to scale. The first ceramic layer 17 isrelatively thin and is deposited on the first electrode 12 with athickness between 1-10 microns. The first ceramic layer 17 is typicallysintered to full density under reducing atmosphere. The reducingatmosphere can be selected from a variety of gases. For example, thereducing atmosphere may be selected from: Ar and H₂ gas with thehydrogen concentration typically between 1-10%. Other gases like N₂/H₂can be used. The reduction temperature is typically between 900-1400degrees C. The typical time duration at peak temperature is typically5-60 minutes. It is understood that a variety of temperature profilescan be utilized. The first ceramic layer 17 preferably has aconductivity that is higher (i.e., more conductive) than 5 ohm-cm afterthe reducing sintering treatment.

The porous ceramic body 18 is deposited as a second ceramic layer ontothe first ceramic layer 17. It is also understood that the porousceramic body 18 can be deposited directly on the first electrode 12.Both the first ceramic layer 17 and the porous ceramic body 18 can bedeposited using conventional ceramic deposition techniques such asscreen printing, doctor blade, lamination, spraying or dip coating. Inthe current example, the first ceramic layer 17 and the porous ceramicbody 18 are deposited using Electrophoretic Deposition (EPD).

Since the first ceramic layer 17 is semi-conductive after the reductionsintering, a second layer (e.g., forming porous ceramic body 18) can bedeposited using EPD techniques. In general, EPD uses electricallycharged particles that are deposited on a counter electrode under theinfluence of an electric field. EPD can deposit wide range ofparticulate materials, almost without dependency on their chemicalcomposition. EPD can form free standing bodies and layers. The layerscan be from micron range thick to a few mm in thickness. The solidloading of the powder in the suspension, electric field intensity andtime are the main parameters to control weight of deposition in an EPDprocess.

The porous ceramic body 18 is constructed from high surface area e.g.,BET (Brunauer, Emmett, Teller) of 0.5-5 m2/g powders. The porous ceramicbody 18 is typically sintered under reducing atmosphere such as thereducing atmosphere disclosed above in connection with the first ceramiclayer. A variety of reduction temperatures can be utilized. Thereduction temperature is typically selected to maintain the open porestructure and avoid massive densification. The thickness of the porousceramic body 18 after reduction is typically in the 10-250 micron range.

The resulting ceramic body has an open pore structure with porosity upto 75% of volume. The pore structure can include a wide range of poresizes from 0.1-6 microns. Preferably pore sizes are in the 0.3-3 micronrange. The porous ceramic body 18 can have a variety of geometries thatwill typically depend on the shape of the first electrode 12. Forexample, the first electrode 12 can be rectangular having a thickness ofapproximately 100 microns. The porous ceramic body 18 can be depositedon the first electrode 12 from multiple angles (e.g., from two sides).The first electrode can have other geometric profiles such as combshapes, and/or simple or complex shapes including one or more polygons.

Returning to FIG. 1, dielectric layer 22 is formed on the porous ceramicbody 18 (and first ceramic layer 17 if present). It is noted thatreference numbers 22 and 18 as shown in the drawings point to the samestructure. This is due to the difficulties in illustrating the variousthin layers in the drawings. It is understood that the dielectric layer17 generally overlies the free surface area of the porous ceramic body18. The resulting shape of the porous ceramic body remains essentiallyunchanged after deposition of the dielectric layer (except for a verysmall increase in thickness). The dielectric layer 22 can be formed viaa variety of processes including thermal or electrochemical oxidation.In an embodiment using thermal oxidation the porous ceramic body 18 isheated in an oxygen containing atmosphere to temperatures between500-1400 degrees C. for periods of 5-120 minutes. This leads to theformation of a thin dielectric layer on the semi-conductive porousceramic body 18 (and first ceramic layer 17 if present). Pump and purgecycles can be used during the treatment and/or at peak temperature toinsure oxygen presence within the pores of the porous ceramic body 18.

In an embodiment using electrochemical oxidation, the porous ceramicbody 18 (and first ceramic layer 17 if present) is inserted in one ormore high base solutions, alkaline, (pH>10). This leads to the formationof a dielectric layer. For example, the solution can be ammonium based,St, BaOH2 (Barium, strontium Hydroxide) optionally in combination withBr, St 8(OH)₂. The porous ceramic body 18 is dipped into a solution thatis typically heated to temperatures between 40-130 degrees C. Ananodizing electric field is typically applied causing adissolution-reduction to occur at the free surface of porous ceramicbody 18. Typical apply voltages are between 2-20V, for a period of 1-12hrs. Once the dielectric layer is formed, a post treatment can beutilized to stabilize the dielectric layer 22 (e.g., heating to 250-1200degrees C. in an oxygen atmosphere).

The properties of the dielectric layer can be further modified bydissolving elements such as Mn, Nb, Mg, Si, Zr, Ti, Bi, Cu, Ag in thesolution. These elements participate in the electrochemically formattedoxidized layer. Post treatment to 1200 degrees C. can also be applied.For example, a highly stable dielectric layer of manganese layer (MnxOy,x>2, y>3) can be formed by oxidation and post heating treatment. Thedielectric layer 22 (whether formed by thermal or electrochemicaloxidation or other processes) is typically formed with a thickness inthe 0.1-1 micron range and has a high insulation resistance (IR)typically in the 10⁶-10¹¹ ohm range. Once oxidized, the porous ceramicbody 18 (and first ceramic layer 17 if present) is typicallycharacterized by high dielectric constant (K) in the 500-50000 range.

A conductive medium 16 is used to fill the porous ceramic body 18.Various compositions can be used as a conductive medium includingconductive polymers: PEDT (poly(3,4-ethylenedioxythiophene),poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, polyanilines,polythiophenes, poly(p-phenylene sulfide), and poly(para-phenylenevinylene)s (PPV) and its soluble derivatives, poly(3-alkylthiophenes),polyindole, polypyrene, polycarbazole, polyazulene, polyazepine,poly(fluorene)s, and polynaphthalene.

In one embodiment, a commercially available PEDT composition is utilized(Clevios (PEDT) available from HC Starck Inc. of Newton, Mass.). Ingeneral the conductive medium should have a conductivity greater than100 siemens/cm and preferably equal or higher than 1000 s/cm. Theconductive medium can be applied by a variety of processes including dipcoating impregnation. In general the porous ceramic body 18 isimpregnated with a diluted mixture containing fine dispersed particlesof the conductive medium (e.g., nano-sized).

In general, the conductive medium 16 forms second electrode. Subsequentstructure can then be added to encapsulate the capacitor structure andform suitable mounting structures and/or electrical terminations. Inthis example, a conductive metal layer 20 encapsulates the porousceramic body 18 (now coated with a dielectric layer 22 and a conductivemedium 16) and functions as an electrical termination for the secondelectrode. A variety of metals can be used. In one embodiment a highlyconductive silver paste is used. The metal layer can be can be appliedby a variety of techniques including brushing or dipping. Suitablemetals are commercially available from a variety of sources includingLord Corporation of Cary, N.C., Emerson and Cumming of Irvine Calif.,and Du-Pont.

FIG. 3 shows another embodiment in accordance with the invention. Theindividual layers shown in FIG. 3 can be formed using the materials andprocesses discussed above in connection with FIG. 1. In this embodiment,the capacitor 100 has a first electrode 212 comprising in this example ametal foil. A first ceramic layer 217 is disposed on the metal foil 212.A porous ceramic body 218 is formed on the first ceramic layer 217. Itis understood that the porous ceramic body 218 can be directly formed onthe first electrode 212. A dielectric layer 222 is formed on the porousceramic body 218. A conductive medium 216 encapsulates the porousceramic body 218. The conductive medium 216 essentially forms a secondelectrode. Suitable conductive medium can include the conductivepolymers discussed above in connection with FIG. 1.

It is noted that reference numbers 222, 218 and 216 as shown in thedrawings point to the same structure. This is due to the difficulties inillustrating the various thin layers in the drawings. It is understoodthat the dielectric layer 222 generally overlies the free surface areaof the porous ceramic body 218. The resulting shape of the porousceramic body remains essentially unchanged after deposition of thedielectric layer (except for a very small increase in thickness). Theconductive medium 216 then encapsulates the porous ceramic body yieldingthe generally rectangular shape shown in FIG. 3.

The remaining structure is primarily directed towards forming electricalterminations and encapsulating the resulting structure. In this example,at least a portion of the conductive polymer layer is covered in ahighly conductive silver paste layer 260. A first electrical lead 270 iselectrically coupled to the first electrode 212 and a second electricallead 280 is coupled to the silver paste layer 260. The structure is alsoencapsulated by layer 290 which can comprise a variety of materials suchas epoxy, resin, parylene or a variety of known molding techniques. Theentire capacitor assembly can be attached to a mounting substrate 295 asdiscussed in more detail below.

FIG. 4 shows a detailed view of a mounting substrate 295 such as atypical printed circuit (PC) board. The mounting substrate includes abase portion 300 which can be fabricated using conventional PC boardtechnologies. A first and second metallization 302, 304 are formed onthe substrate 300. The first and second metallization 302, 304 aregenerally spaced apart so as to align with the first and secondelectrical leads 270, 280 as shown in FIG. 3. It is understood thatfirst and second metallizations can be coupled to one or more circuittraces (not shown). Conductive glue 306 is used to electrically couplethe first and electrical leads 270, 280 to the first and secondmetallization 302, 304. A non-conductive glue 308 can be used in thearea disposed between the first and second metallizations 302, 304. Oncethe capacitor 100 is mounted, to the substrate a soldering operation canfollow using conventional techniques. It is understood that a widevariety of mounting techniques can be used without departing from thescope of the invention.

FIG. 5 shows another embodiment in accordance with the invention. Inthis embodiment, the first electrode 512 is disposed along the edge ofthe structure as opposed to the centrally disposed first electrode 212disclosed in FIG. 4. The individual layers shown in FIG. 5 can be formedusing the materials and processes discussed above in connection withFIG. 1. In this embodiment, the capacitor 400 has a first electrode 512.A first ceramic layer 517 is disposed on the metal foil 512. A porousceramic body 518 is formed on the first ceramic layer 517. It isunderstood that the porous ceramic body 518 can be directly formed onthe first electrode 512,

A dielectric layer 522 is formed on the free surface of the porousceramic body 518. A conductive medium 516 (e.g., conductive polymer) isimpregnated into the pores of the porous body 518.

It is noted that reference numbers 522, 518 and 516 as shown in thedrawings point to the same structure. This is due to the difficulties inillustrating the various thin layers in the drawings. It is understoodthat the dielectric layer 522 generally overlies the free surface areaof the porous ceramic body 518. The resulting shape of the porousceramic body remains essentially unchanged after deposition of thedielectric layer (except for a very small increase in thickness). Theconductive medium 522 then encapsulates the porous ceramic body yieldingthe generally rectangular shape shown in FIG. 5.

The typical thickness of layer 516 is 1-10 microns. Full impregnation ofall open pores in the porous ceramic body 518 is preferred as thisprovides maximum capacitance. The conductive polymer preferably has goodadhesion to the dielectric layer 522 to insure good electricalperformance. An insulating layer 513 is disposed between the firstelectrode and the remaining layers that are coupled to the secondelectrode to provide electrical insulation between the two electrodes. Awide variety of insulating materials can be used for this purpose.

The remaining structure is primarily directed towards forming electricalterminations for the second electrode and encapsulating the resultingstructure. A conductive metal paste layer 560 (e.g., silver paste) isapplied to a portion of the conductive medium 516. A first electricallead 570 is electrically coupled to the first electrode 512 and a secondelectrical lead 580 is coupled to the conductive metal paste layer 560.The structure is also encapsulated by layer 590 which can comprise avariety of materials such as epoxy, resin, parylene applied by one of avariety of known techniques. It is understood that a variety ofelectrical terminations and encapsulation techniques can be utilizedwithout departing from the scope of the invention. It is also understoodthat additional layers could be added to the structure without departingfrom the scope of the invention.

FIG. 6 illustrates one embodiment of a method of manufacturing a bulkcapacitor of the present invention. The various process steps set forthin FIG. 6 are carried as discussed generally above. In step 600 asuitable ceramic material is prepared. The ceramic particles are thenplaced in a suitable suspension for deposition (e.g., by EPD) as shownby block 602. The first electrode (e.g., Kovar foil) is shaped into thedesired geometric shape using conventional methods as shown by block604.

A first (thin) ceramic layer is then deposited on the first electrode asshown by block 606. The first ceramic layer is then sintered to fulldensity under a reducing atmosphere (e.g., Ar/H₂) as shown by block 608.The reduction temperature is typically between 900-4400 degrees C. Thetypical time duration at peak temperature is typically 5-60 minutes.Next a porous ceramic body is deposited on the first ceramic layer asshown by block 610. The porous ceramic body is then sintered in reducingatmosphere as shown by block 612 and as discussed generally above. Avariety of reduction temperatures can be utilized. The reductiontemperature is typically selected to maintain the open pore structureand avoid massive densification.

The porous ceramic body is then oxidized to form a thin dielectric layeras shown by block 614. For example, the porous ceramic body can be isheated in an oxygen containing atmosphere to temperatures of 900 degreesC. for approximately 5-120 minutes. This leads to the formation of adielectric layer. Pump and purge cycles can be used during the processto insure oxygen presence within the pores of the porous ceramic body.

The porous ceramic body (now covered in a thin dielectric layer) is thenimpregnated with a conductive medium (e.g., a conductive polymer asdiscussed above) as shown by block 616. In this example, at least aportion of the porous ceramic body is then coated with a suitable silvercompound (e.g., silver paste) as shown by block 618. Electrical leadsforming the anode and cathode are attached and the package isencapsulated as shown by block 620.

A bulk capacitor and methods of manufacturing the bulk capacitor havebeen disclosed. That which has been disclosed is merely exemplary. Thepresent invention contemplates numerous variations, options, andalternatives fall within the spirit and scope of the claimed invention.Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above described embodiments without departing from the spirit andscope of the invention, and that such modifications, alterations, andcombinations are to be viewed as being within the ambit of the inventiveconcept. It is intended in the appended claims to cover all thosechanges and modifications which fall within the true spirit and scope ofthe present invention.

1. A bulk capacitor, comprising: a first electrode formed of a metalfoil, a semi-conductive porous ceramic body deposited on the metal foil,a dielectric layer formed on the porous ceramic body, and a conductivemedium filling at least a portion of porous ceramic body forming asecond electrode.
 2. The bulk capacitor of claim 1 further comprising aconductive metal layer encapsulating the porous ceramic body.
 3. Thebulk capacitor of claim 2 wherein the conductive metal layer comprisessilver.
 4. The bulk capacitor of claim 1 further comprising asemi-conductive ceramic layer disposed between the metal foil and theporous ceramic body.
 5. The bulk capacitor of claim 1 wherein the metalfoil has a geometric profile.
 6. The bulk capacitor of claim 1 whereinthe conductive medium comprises a conductive polymer.
 7. The bulkcapacitor of claim 1 wherein the dielectric layer is formed on a freesurface of the porous ceramic body.
 8. The bulk capacitor of claim 1wherein the dielectric layer is formed on porous ceramic body and on themetal foil.
 9. The bulk capacitor of claim 1 further comprising a firstelectrical lead coupled to the first electrode and a second electricallead coupled to the second electrode, wherein the first electrical leadand second electrical lead are configured to mate with first and secondconductive pads located on a mounting substrate.
 10. The bulk capacitorof claim 9 wherein the first and second electrode are joined to thefirst and second conductive pads via a conductive adhesive.
 11. The bulkcapacitor of claim 10 further comprising a non-conductive adhesivedisposed between the first and second electrical leads.
 12. A method ofmanufacturing a bulk capacitor, the method comprising: forming aconductive porous ceramic body on a first electrode comprising a metalfoil, oxidizing the porous ceramic body to form a dielectric layer, andfilling the porous ceramic body with a conductive medium forming asecond electrode.
 13. The method of claim 12 comprising encapsulatingthe porous ceramic body with a conductive metal layer.
 14. The method ofclaim 13 wherein the conductive metal layer comprises silver.
 15. Themethod of claim 12 further comprising forming a semi-conductive ceramiclayer between the metal foil and the porous ceramic body.
 16. The methodof claim 12 comprising forming a geometric profile in the metal foil,17. The method of claim 12 wherein the oxidizing is performed thermally.18. The method of claim 12 wherein the oxidizing is performedelectrochemically.
 19. The method of claim 12 wherein the conductivemedium comprises a conductive polymer.
 20. The method of claim 12wherein the dielectric layer is formed on porous ceramic body and on themetal foil.
 21. The method of claim 12 wherein a first electrical leadis coupled to the first electrode and a second electrical lead iscoupled to the second electrode, wherein the first electrical lead andsecond electrical lead are configured to mate with first and secondconductive pads located on a mounting substrate.
 22. The method of claim21 wherein the first and second electrode are joined to the first andsecond conductive pads via a conductive adhesive.
 23. The method ofclaim 22 wherein a non-conductive adhesive is disposed between the firstand second electrical leads.